Conventionally, a piston of a reciprocating engine is provided with, in addition to a compression ring for keeping combustion gas out, an oil ring for keeping lubricating oil out.
The oil ring is classified into a three-piece type used mainly in a gasoline engine and a two-piece type used mainly in a diesel engine. To meet a demand for low fuel consumption, however, the two-piece type oil ring, which may have a smaller width in an axial direction, has been increasingly used in the gasoline engine.
As the two-piece oil ring, there is known an oil ring including two components: an oil ring body having a web portion provided with an oil passage hole and a pair of rail portions integrally formed with the web portion at axial ends (a top and a bottom) of the web portion; and a coil expander mounted on a radially inner side of the oil ring body to push the oil ring body radially outward. In this case, the oil ring body is formed in a split ring shape with an opening so as to be able to extend (to increase a diameter of the oil ring body) when pushed radially outward by the coil expander. When pushed by the coil expander, the oil ring body extends in such a manner that a slide-contact surface of each of the rail portions facing radially outward contacts with an inner surface of a cylinder and applies a certain contact pressure (a surface pressure) to the inner surface. At this time, the slide-contact surface, in accordance with reciprocation of the piston, applies the oil retained between the pair of rail portions to the inner surface of the cylinder while the rail portion scrapes off excess oil and sends the excess oil back to a crank chamber via the oil passage hole. Thereby, an oil film with an appropriate thickness is formed on the inner surface of the cylinder.
In recent years, following an improvement in functionality of an internal combustion engine to meet a market demand for the low fuel consumption and low oil consumption, there has been a demand for an oil ring capable of reducing friction against the inner surface of the cylinder as well as the oil consumption by controlling an oil scraping-up action during piston upstroke and amplifying an oil scraping-off action. In an effort to meet such a demand, there are proposed oil rings having peripheral surfaces in various shapes facing radially outward.
For example, PLT 1 set forth below describes an oil ring having, on respective peripheral portions of an upper rail portion and an lower rail portion close to the combustion chamber, tapered portions gradually decreasing in diameter toward the combustion chamber.